DrunkWalker creates some of the most gawdawful JS code you'll ever see.
That email validation that is part of the code you show is, for example, so inadequate it is worthless. Ditto its validation of text fields. And, as you noted, its validation of numbers is nearly pointless. Doesn't even allow you to insist on integers, for example.
It's way way past time to get rid of DreadedWeasel code whenever possible.
Example to meet your specs:
function checkQuarters( fld )
var val = parseFloat(fld.value);
if ( isNaN(val) || ( val*4 != Math.floor(val*4) ) )
alert("Value must be a multiple of 0.25");
Amount, to nearest 25 cents:
<input name="whatever" onchange="checkQuarters(this);" />